The long-term goal of this research is to study the role of aging on the differentiation of sensory neurons within the central nervous system. A special class of vestibular neurons, the principal cells of the chick tangential nucleus, are used as a prototype. Using light microscopic methods, the development of principal cells and their major afferent input, the spoon endings, was documented in normal and in otocyst-ablated embryos and these findings are the basis for continued ultrastructural studies on this system. Ultrastructural studies on spoons and principal cells in embryos, hatchlings, and young adults have demonstrated major developmental changes in synaptic endings and synaptic membrane complexes. Future studies will be directed toward a comparison of ultrastructural features of principal cells in normal and otocyst-ablated embryos.